Duplicate Python versioning in Travis CI

[dgasmith]

Current in Travis we set:

    - os: linux
      python: 3.7
      env: PYTHON_VER=3.7

This will both build a conda env and use a system 3.7 python. There is no need to set the Python version I believe, we may want to set the name= variable instead so that the display is correct.

We might also want to unpin the xenial container.

[dgasmith]

This was fixed sometime ago.